A photographic process for forming a direct positive image without the necessity of either a reversal processing step or a negative film is well known.
The method of forming a positive image by the use of a conventional known direct positive silver halide photographic material may be classified into the following two groups, with certain exceptions, in considering the practical usefulness thereof.
One group uses a previously fogged silver halide emulsion, in which the fogged nuclei (latent image) in the exposed area are broken by solarization or the Herschel effect and the intended positive image is directly obtained by development.
The other group uses a non-fogged internal latent image-type silver halide emulsion, in which surface development is effected after imagewise exposure and after or during fogging to obtain a direct positive image.
An internal latent image-type silver halide photographic emulsion as referred to herein means a silver halide photographic emulsion of a type such that the silver halide grains in the emulsion have light-sensitive nuclei essentially in the inside thereof and therefore a latent image is formed essentially in the inside of the grains by exposure.
The method of the latter type generally provides a higher sensitivity than that of the former type and is therefore suitable for uses requiring high sensitivity. The method of the present invention belongs to the method of the latter type.
In this technical field, various techniques are known, for instance, as disclosed in U.S. Pat. Nos. 2,592,250, 2,468,957, 2,497,875, 2,588,982, 3,317,322, 2,497,875, 3,761,266, 3,761,276, 3,796,577, and British Patents 1,151,363, 1,150,553 and 1,011,062. Using the disclosed known methods, direct positive photographic materials having a relatively high sensitivity can be obtained.
On the other hand, the details of the mechanism of forming direct positive images are described, for example, in T.H. James, The Theory of the Photographic Process, 4th Ed., Chap. 7, pages 182 to 193 and U.S. Pat. No. 3,761,276.
More specifically, it is believed that fogged nuclei are selectively formed on only the surfaces of the silver halide grains in the non-exposed area due to the surface desensitization action occurring due to a so-called internal latent image formed in the inside of the silver halide by the first imagewise exposure and thereafter a photographic image (direct positive image) is formed in the non-exposed area by the subsequent surface development.
As mentioned above, means of selectively forming fogged images, in general, include a so-called "light-fogging method" where a second exposure is imparted to the complete surface of a light-sensitive layer (for example, British Patent 1,151,363) and a so-called "chemical fogging method" where a nucleating agent is used. The latter method is described, for example, in Research Disclosure, Vol. 151, No. 15162 (issued on November, 1976), pages 72 to 87.
For forming a direct positive color image, an internal latent image-type silver halide photographic material is subjected to surface color-development, after or during fogging, and thereafter it is bleached (or bleach-fixed). After bleaching and fixation, the material is then generally rinsed in water and/or stabilized.
On the other hand, JP-A-62-150241 and JP-A-62-275242 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") illustrate direct positive photographic materials having a low-sensitivity emulsion containing cubic or essentially (100) face tetradecahedral silver halide grains. These publications disclose photographic materials forming black-and-white direct positive images, in which the exposure latitude of forming black-and-white direct positive images and the graininess of the images formed are improved.
With respect to the above-mentioned method of forming a direct positive image by the use of a non-fogged internal latent image-type silver halide emulsion, various techniques for increasing the maximum image density and decreasing the minimum image density have heretofore been proposed, but a sufficiently improved technique has not been attained at the present time. In particular, the minimum image density would often increase, depending upon the kind of the sensitizing dye used and the method of sensitizing the layer, in a sensitizing dye-containing red-sensitive layer or green-sensitive layer. Therefore, the improvement of the color reproducibility to be attained by optimized color sensitization and the reproduction of the whiteness in the white background areas of the image to be attained by a decrease in the minimum image density would often be contradictory to each other.
The above-mentioned chemical fogging method using a nucleating agent is grouped into a case where the nucleating agent is incorporated in a photographic material and a case where it is incorporated in a color developer. However, the former case has a problem, that the minimum image density increases or the maximum image density is decreased during storage of the photographic material.
Where the emulsion as described in the above-mentioned JP-A-62-150241, which contains cubic or essentially (100) face tetradecahedral silver halide grains, is employed in the formation of color images, an improvement in the color reproducibility by optimization of color sensitization could be attained relatively easily but the direct positive color image obtained still does not have a satisfactorily high maximum image density and a satisfactorily low minimum image density despite the improved color reproducibility. Accordingly, a solution to the problem is desired.